1. Field of the Invention
The present invention relates to a zoom lens including a variable focus lens.
2. Description of the Related Art
Conventionally, a variable focus lens (variable focus element) that is capable of changing a refractive power by controlling the shape of a liquid interface is known. The variable focus lens disclosed in Japanese Patent Laid-Open No. 2006-178469 can deform the contact surface (interface) formed between a conductive liquid and an insulation liquid having different refraction indices, respectively. The zoom lens disclosed in Japanese Patent Laid-Open No. 2008-541184 employs two variable focus lenses. The zoom lens disclosed in Japanese Patent Laid-Open No. 2007-518130 employs one variable focus lens and one switchable variable focus lens. The switchable variable focus lens changes the refractive power for the entire elements by providing a cavity on the exit side of a solid lens and exchanging gas and liquid within the cavity.
In the zoom lens disclosed in Japanese Patent Laid-Open No. 2008-541184 and Japanese Patent Laid-Open No. 2007-518130, a variable focus lens is disposed closest to the object (subject) side of the optical system. However, a variable focus lens is susceptible to the external influence by being disposed closest to the object side of the optical system. For example, when a variable focus lens is exposed to direct sunlight, the temperature difference between the shadowed portion and the exposed portion to sunlight occurs, resulting in the occurrence of convection in the liquid inside thereof. With this arrangement, the optical characteristics of the variable focus lens become unstable. This effect is considerably increased when the variable surface of the variable focus lens becomes larger.